That's very different. This is functionally checking a flag[1], and acting accordingly. There's nothing to fail or make succeed. [1] There's an ioctl involved,

That's very different. This is functionally checking a flag[1], and acting accordingly. There's nothing to fail or make succeed. [1] There's an ioctl involved, and it can fail for other reasons than "fd is not terminal", however all the reason for the operation to fail are good reasons to not proceed with terminal specific behavior.